A unified theory of thin film and bulk bilayer nickelates

Summary

This study proposes a unified theory based on a two-component model to explain a series of key experimental phenomena in pressurized bulk and thin-film bilayer nickelate superconductors. Centered on the interlayer superexchange coupling and hybridization between strongly correlated localized electrons and itinerant electrons at the nickel orbitals, the theory predicts two distinct behaviors of the superconducting transition temperature with doping: when the interlayer superexchange coupling is strong, electron or hole doping respectively produce two superconducting domes, with a non-superconducting interlayer valence-bond state appearing near half-filling; when the coupling is weak or moderate, the two domes merge into a single dome that spans half-filling but has a lower maximum temperature. Increasing doping drives the normal state from a Fermi liquid to a non-Fermi liquid or weakly insulating state, with a quasi-linear resistivity scattering rate emerging near optimal doping. Oxygen vacancies or chemical substitutions can disrupt the interlayer valence bond, simultaneously suppressing superconductivity and inducing local Kondo scattering of itinerant electrons, which explains the logarithmic temperature dependence of resistivity and the negative magnetoresistance observed in non-superconducting samples. This framework uniformly accounts for the differences in superconducting transition and normal state between bulk and thin films, the effects of hole doping and oxygen stoichiometry on the dome shape, and the competitive relationship between superconductivity and the Kondo effect. Based on the theory, the authors propose that bulk superconductivity at ambient pressure can be achieved through doping or by reducing the interlayer magnetic coupling, and predict that electron doping will yield higher transition temperatures.

Materials

• bulk La₃Ni₂O₇
• La₃Ni₂O₇
• La₂PrNi₂O₇
• (La,Pr,Sm)₃Ni₂O₇
• (La,Si)₃Ni₂O₇
• (La,Pr)₃Ni₂O₇
• La₃Ni₂O₇-δ
• Sr-doped (La,Pr)₃Ni₂O₇

Methods

• two-orbital t-J model
• slave particle representation
• self-consistent solutions
• Bethe-Salpeter equation

Keywords

• high temperature superconductivity
• interlayer superexchange coupling
• valence bond state
• kondo effect
• non fermi liquid
• d wave pairing
• superconducting domes
• ambient pressure superconductivity

Highlights

• A unified framework reconciles disparate superconducting and normal-state behaviors in pressurized bulk and thin-film bilayer nickelates.
• The valence bond state at dz2 half-filling acts as a non-superconducting phase that separates or connects the superconducting domes depending on interlayer coupling strength.
• Breaking of interlayer valence bonds simultaneously destroys superconductivity and produces Kondo scattering, explaining Kondo effect in non-superconducting samples.
• Ambient-pressure superconductivity is predicted through tuning of interlayer distance or doping.
• The universal linear relation between T_c and the c-axis lattice constant highlights the crucial role of interlayer superexchange.

Conclusions

• The two-component scenario with strongly correlated dz2 electrons and itinerant dx2-y2 electrons explains key experimental observations in bilayer nickelates.
• For strong interlayer superexchange coupling (bulk), two superconducting domes upon electron and hole doping are separated by a valence bond state near half filling; for weak coupling (thin films), a single dome spans half filling with lower T_c.
• Increasing doping drives the normal state from Fermi liquid to non-Fermi liquid to weakly insulating, with quasi-linear-in-T scattering near optimal T_c.
• Oxygen vacancies or nonmagnetic substitution break interlayer valence bonds, suppressing superconductivity and inducing local Kondo scattering.
• Ambient pressure bulk superconductivity is predicted via doping or reducing interlayer magnetic coupling, with higher T_c possible upon electron doping.

Main claims

• The two-component model with interlayer superexchange coupling and hybridization can unify the key experimental observations of bilayer nickelates.
  • Evidence: Abstract: 'We provide a unified explanation based on the two-component scenario for a number of key experimental observations reported recently.',Full text: 'This work provides a unified theory that can satisfactorily address all above observations.'
• For strong interlayer coupling (bulk), electron and hole doping produce two separate superconducting domes with a non-superconducting valence bond state near half filling.
  • Evidence: Full text: 'For large J with strong pairing strength, we obtain two separate superconducting domes on electron and hole doping regions… The suppression of T_c… Close to half filling… T_c rapidly drops to zero, suggesting… non-superconducting valence bond state (VBS).'
• In thin films with weaker interlayer coupling, the two domes merge into a single dome with lower maximum T_c, explaining the experimental observations.
  • Evidence: Full text: 'For smaller or moderate J, the VBS is also weakened… and can now couple with electrons to form superconductivity. This leads to a finite T_c around half filling that merges the two superconducting domes into a single one.'
• Oxygen vacancies break interlayer valence bonds, suppress superconductivity, and induce Kondo scattering, explaining the observed Kondo effect in non-superconducting samples.
  • Evidence: Full text: 'When there exist inner apical oxygen vacancies… the VBS is destroyed to produce one or two decoupled Ni-dz2 spins, whose hybridization with surrounding electrons can induce effective Kondo scattering… Numerically, one can simulate this by setting the local J to zero…. We see a sharp peak in ImSigma and a dip in the DOS at zero frequency, a clear indication of Kondo resonance.'
• Doping drives the normal state from Fermi liquid to non-Fermi liquid with linear-in-T scattering near optimal T_c, and further to weakly insulating, consistent with transport experiments.
  • Evidence: Full text: 'Close to half filling, it vanishes at low temperatures… indicating a Fermi liquid normal state. Increasing hole (electron) doping quickly suppresses the VBS gap and enhances the inter-orbital scattering, causing a NFL normal state with quasi-linear-in-T behavior… A WI region emerges where ImSigma increases logarithmically with decreasing temperature.'
• The theory predicts that bulk superconductivity at ambient pressure can be achieved by doping or reducing interlayer magnetic coupling, and electron doping will yield even higher T_c.
  • Evidence: Abstract: 'We propose bulk superconductivity at ambient pressure by doping or reducing the interlayer magnetic coupling and predict even higher T_c upon electron doping.',Full text: 'Our theory has two immediate predictions: 1) ambient pressure superconductivity by doping or reducing the interlayer magnetic coupling…; 2) a second superconducting dome upon electron doping with an even higher maximal T_c.'

Workflow

• model_formulation — The two-component model captures the essential physics of bilayer nickelates, with interlayer superexchange coupling as a key parameter.
  • Materials: two-orbital t-t_perp-J model; slave particle representation
  • Methods: definition of Hamiltonian with interlayer superexchange J, hybridization V, and Coulomb repulsion U; slave particle decomposition with spinon, holon, doublon operators
  • Observations: effective interlayer pairing of dz2 electrons; hybridization between localized dz2 and itinerant dx2-y2 electrons
• superconductivity_and_VBS_calculations — Interlayer superexchange strength controls the number of superconducting domes and the appearance of an intermediate valence bond state.
  • Materials: Bethe-Salpeter equation; normal state Green's functions
  • Methods: solving pairing vertex divergence to determine T_c; analyzing momentum dependence of pairing gap
  • Observations: two separate superconducting domes for strong J, separated by non-superconducting valence bond state near half filling; single superconducting dome for moderate J; anisotropic s+–wave pairing on bonding/antibonding bands, isotropic s-wave on dz2 pocket; maximum T_c at optimal doping
• normal_state_and_kondo_analysis — Doping and oxygen vacancies control the normal state from FL to NFL/WI and induce Kondo scattering in non-superconducting samples.
  • Materials: self-energy of dx2-y2 electrons; density of states
  • Methods: calculation of imaginary part of self-energy to obtain scattering rate; simulation of broken VBS by setting local J=0 to mimic oxygen vacancies
  • Observations: Fermi liquid behavior near half filling with VBS gap; non-Fermi liquid with quasi-linear-in-T resistivity near optimal doping; weakly insulating state at higher doping with log-T resistivity; Kondo resonance peak in self-energy and dip in DOS when VBS broken
• unified_phase_diagram_and_predictions — The theory predicts ambient pressure bulk superconductivity by doping or reducing interlayer coupling, and a second superconducting dome under electron doping with potentially higher T_c.
  • Materials: combined computational results
  • Methods: synthesis of T_c, normal state, and VBS/Kondo results into a global phase diagram
  • Observations: phase diagram showing superconducting domes, VBS, FL, NFL, WI, Kondo, and magnetic regions; unified explanation for differences between bulk and thin films